PREPARATION OF ACTIVE HDS CATALYSTS BY CONTROLLING THE DISPERSION OF ACTIVE SPECIES

It is demonstrated that the structural control of the metal ion precur sors in the impregnating solution by adding the chelating agents is ef fective to prepare the higher active Go-Mo supported on alumina cataly sts (Co-Mo/Al2O3) for hydrodesulfurization (HDS). Coordination structu res of the Co and Mo complexes in the Go-Mo impregnating solution and distributions of the Co and Mo complexes were evaluated by spectroscop ic characterization techniques and by using a computational calculatio n, respectively. An addition of a chelating agent, such as NTA (nitril otriacetic acid); and Glu (L-glutamic acid), in the Go-Mo solution res ults in the selective formation of the Co complexes, while the amount of the Mo complex is negligibly small at the practical DH of 9.2. The addition of the chelating agent increases the thiophene HDS activity o f the sulfided catalysts typically by 50%, compared with that prepared without the chelating agent. Dispersion results of Co and Mo species on both oxidic and sulfided catalysts indicate that the higher HDS act ivity is explained by the higher degree of surface exposure of Co site s (namely the dispersion of Co) rather than that of Mo sites. The sele ctive formation of the Co-chelate complexes keeps Co ions stable in so lution up to high concentration. Furthermore, the Co complexes are est imated to be stable on the support even in the initial step of calcina tion, which would depress the formation of crystalline Co compounds, s uch as CoAl2O4 and CoMoO4. These effects result in the higher dispersi on of the active Co surface species. (C) 1997 Elsevier Science B.V.